1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device, which is low in profile, small in occupied volume, and lightweight, has been extensively used in electric products including portable telephones and notebook computers. However, since the liquid crystal display device does not emit light by itself, it is necessary to provide a separate illuminating apparatus besides the liquid crystal display device when used in dark places where the brightness of the sunlight and the illumination in room is not fully available. Thus, the illuminating apparatus to irradiate liquid crystals is desired to be compact in size and small in power consumption, and in recent years, a low profile spread illuminating apparatus of side light system (the light conductive plate system) is often used. The inventors of the present invention proposed a conventional spread illuminating apparatus disclosed in the Japanese Unexamined Patent Application Publication No. 2000-11723.
FIG. 7 shows an embodiment of the conventional spread illuminating apparatus.
As shown in the figure, a spread illuminating apparatus 1xe2x80x2 generally comprises a transparent substrate 2 made of a light-transmissible material, and a light source 5 provided close to an end surface 8 of the transparent substrate 2. Light emitted from the light source 5 is guided into the transparent substrate 2 to irradiate a liquid crystal display device (not shown in the figure) disposed on a lower side of the transparent substrate 2.
The light source 5 comprises a light conductive member 3 made of a transparent material and shaped bar-like, and a spot-like light source (for example, a light-emitting diode) 4 disposed on one end of the light conductive member 3. The light conductive member 3 is provided with an optical path conversion means 12. This optical path conversion means 12 is formed with grooves, for example triangular in section, provided on a surface of the light conductive member 3 opposite to a surface 9 facing an end surface 8 of the transparent substrate 2 and thereby has a function to allow light emitted from the spot-like light source 4 to be made incident substantially uniformly on the end surface 8 of the transparent substrate 2. The light conductive member 3 is disposed at a prescribed distance from the end surface 8 of the transparent substrate 2 with the surface 9 running parallel to the end surface 8.
In order to efficiently introduce the light emitted from the spot-like light source 4 into the transparent substrate 2, a light reflection member (frame) 13 is provided around the light conductive member 3. The light reflection member 13 is substantially U-shaped and covers the longitudinal periphery of the light conductive member 3 except the surface 9 facing the transparent substrate 2 (refer to FIG. 8). The light reflection member 13 may have a film on which a metal such as silver is vapor-deposited on its inner surface facing the light conductive member 3, or may be formed of a hard resin with a white film adhered to its inner surface, or a bent metal sheet such as an aluminum sheet, a stainless sheet and the like.
FIG. 8 is a schematic side view showing a light reflection pattern 19 formed on an upper surface 16 of the transparent substrate 2.
The light reflection pattern 19 comprises grooves 17 triangular in section and flat portions 18 adjacent thereto, which are formed at prescribed intervals therebetween in parallel to the light conductive member 3. The grooves 17 are formed to vary in depth from each other so that light coming from the light conductive member 3 and entering the transparent substrate 2 may be reflected in a substantially uniform manner at the whole surface of the transparent substrate 2 irrespective of the distance from the light conductive member 3 (the light source 5) to irradiate the liquid crystal display device (not shown in the figure) provided on the lower side of the transparent substrate 2. In other words, the depth of the grooves 17 increases gradually in proportion to the increase in distance from the light conductive member 3.
FIG. 9 shows another embodiment of a conventional spread illuminating apparatus.
As shown in the figure, a spread illuminating apparatus 1xe2x80x3 comprises a plurality of light sources (5a and 5b) to cope with a larger display screen or to improve the brightness of the screen. The light source 5a is disposed along one end surface 8a of the transparent substrate 2, and the light source 5b is disposed along another end surface 8b opposite to the end surface 8a. Respective lights emitted from spot-like light sources 4a and 4b and entering light conductive members 3a and 3b are uniformly guided to the end surface 8a and the end surface 8b of the transparent substrate 2 by optical path conversion means 12a and 12b formed on one surface of the respective light conductive members 3a and 3b. In order to efficiently introduce the lights emitted from the spot-like light sources 4a and 4b into the transparent substrate 2, light reflection members (frames) 13a and 13b are provided around the light conductive members 3a and 3b, respectively. The light reflection members 13a and 13b are substantially U-shaped and cover the longitudinal peripheries of the light conductive members 3a and 3b except surfaces 9a and 9b facing the transparent substrate 2 (refer to FIG. 10).
FIG. 10 is a schematic side view showing a light reflection pattern 19 formed on an upper surface 16 of the transparent substrate 2.
The light reflection pattern 19 comprises grooves 17 triangular in section and flat portions 18 adjacent thereto, which are formed at prescribed intervals therebetween in parallel to the light conductive members 3a and 3b. The grooves 17 are formed to vary in depth from each other so that lights coming from the light conductive members 3a and 3b and entering the transparent substrate 2 may be reflected in a substantially uniform manner at the whole surface of the transparent substrate 2 irrespective of the distance from the light conductive members 3a and 3b (the light sources 5a and 5b) to irradiate the liquid crystal display device (not shown in the figure) provided on the lower side of the transparent substrate 2. In other words, the depth of the grooves 17 increases gradually in proportion to the increase in distance from the light conductive members 3a and 3b, and is largest at the center of the transparent substrate 2.
Since, in the spread illuminating apparatus of the above-described construction, the grooves (prisms) formed on the surface of the transparent substrate have a triangular shape in section, there is the disadvantage of the undermentioned restrictions in ensuring the visibility of the liquid crystal display image and the workability of the light reflection pattern.
FIG. 3B shows a path of light which is guided into the transparent substrate 2 and is reflected at the grooves 17 of the light reflection pattern 19 to irradiate the liquid crystal display device. When the groove interval Lxe2x80x2 between two adjacent grooves 17 of the light reflection pattern 19 is large, the difference in brightness between a light area irradiated with light reflected at the grooves and a dark area not irradiated with the light reflected at the grooves becomes conspicuous, whereby a liquid crystal display device 10 cannot be irradiated uniformly. And when the groove interval Lxe2x80x2 is set to be more than 0.3 mm, both the depth and width of the grooves 17 also increase to a certain degree, which causes the problem that the grooves 17 become visually recognizable when viewing the liquid crystal display screen. Therefore, the groove interval Lxe2x80x2 must be 0.3 mm or under (preferably, about 0.2 mm).
When the groove interval Lxe2x80x2 is set to be 0.3 mm and the depth of the grooves 17 is changed according to the distance from the light conductive member 3, the depth of the grooves 17 is set to range from about 10 xcexcm to about 25 xcexcm. When the depth of the grooves 17 exceeds 25 xcexcm, the influence by an inclined surface of the grooves 17 emerges giving the problem that the liquid crystal display shows shifted images. Therefore, the maximum depth of the grooves 17 must be set about 20 to 25 xcexcm.
When the groove interval Lxe2x80x2 is set to be 0.15 mm and the depth of the grooves 17 is changed according to the distance from the light conductive member 3, the depth of the grooves 17 is set to range from about 5 xcexcm to about 15 xcexcm. In addition, when the groove interval Lxe2x80x2 is further reduced, the minimum depth of the grooves 17 must be set 5 xcexcm or under, otherwise the shape of the grooves cannot be precisely injection-molded.
Considering these restrictions on the workability, in case of a spread illumination apparatus of conventional construction, the dimension A (equal to the length of the transparent substrate in FIG. 7) of the liquid crystal display screen, through which a uniform illumination can be realized, is restricted to a specific value. For example, the dimension is about 60 mm in the spread illuminating apparatus having one light source disposed close to the transparent substrate 2, and about 120 mm in the spread illuminating apparatus having two light sources.
The present invention has been made in the light of the above problems. Accordingly, it is an object of the present invention to improve a light reflection pattern formed on a substrate, thereby providing a spread illuminating apparatus which can realize a uniform brightness over the whole display screen, an accurate illumination, and an expanded size of a practically usable screen.
In order to solve the above problems, according to a first aspect of the present invention, in a spread illuminating apparatus in which a bar-like light source is disposed close to and along two end surfaces opposing each other of a transparent substrate made of a light-transmissible material, and a light reflection pattern comprising a plurality of grooves and a plurality of flat portions adjacent thereto which are parallel to the length of the light source is formed on the transparent substrate, the grooves on the transparent substrate are substantially inverted-trapezoidal in section.
Light radiated from the light source is guided into the transparent substrate, reflected at inclined surfaces and bottom surfaces of the grooves and the flat portions which together form the light reflection pattern, and irradiates the liquid crystal display screen uniformly. The grooves trapezoidal in section can improve the machinability and durability of a die.
In order to solve the above problems, according to a second aspect of the present invention, in the spread illuminating apparatus according to the first aspect of the present invention, the exterior angle to the inclination angle formed by the inclined surface and the bottom surface of the grooves trapezoidal in section is 35xc2x0 to 55xc2x0. Excellent characteristics of light distribution on an observation screen can be obtained by setting the inclination angle of the inclined surface of the groove forming the light reflection pattern to a prescribed value.
In order to solve the above problems, according to a third aspect of the present invention, in the spread illuminating apparatus according to the second aspect of the present invention, the angles are formed bilaterally symmetric. More excellent characteristics of light distribution can be obtained by making the inclination angles of the right and left inclined surfaces of the grooves bilaterally symmetric.
In order to solve the above problems, according to a fourth aspect of the present invention, in the spread illuminating apparatus according to any one of the first to third aspects of the present invention, the depth of the grooves constituting the light reflection pattern increases in proportion to the increase in distance from the respective light sources, and is largest at the center of the transparent substrate. Still more excellent characteristics of light distribution can be obtained by changing the depth of the grooves of the light reflection pattern.
In order to solve the above problems, according to a fifth aspect of the present invention, in the spread illuminating apparatus according to any one of the first to third aspects of the present invention, the interval between the adjacent grooves constituting the light reflection pattern is in inverse proportion to the increase in distance from the respective light sources and decreases toward the center of the transparent substrate to measure minimum thereat.